There are two basic types of fire hydrants manufactured today and these types are commonly referred to in the trade as the "wet" barrel or "California" fire hydrant and the dry barrel fire hydrant. The wet barrel type of fire hydrant is a hydrant wherein the main hydrant valve is located in the hydrant barrel adjacent the outlet or hose connection for the same and, in situations where there is more than one outlet or hose connection, there is a main hydrant valve for each. This type of hydrant is used in mild climates, such as the southern states of the United States and Hawaii, where there is no chance of freezing. The more prevalent used fire hydrant is of the dry barrel type wherein the hydrant is provided with a single main hydrant valve positioned deep underground below the freeze line to prevent freezing of the hydrant in winter weather.
Since the dry barrel hydrant has its main hydrant valve positioned in the area of the hydrant shoe and the lower end of the barrel below the freeze line, means have been provided for draining the barrel above the main hydrant valve after each use of the fire hydrant so as to maintain the hydrant barrel dry at all times during non-use. These prior means for draining the hydrant barrel have been automatic functioning drain valves designed to work in conjunction with the operation of the main hydrant valve so that a drain passageway from the interior of the barrel to the exterior of the hydrant is either open, when the main hydrant valve is closed, or closed, when the main hydrant valve is open. The drain valves may either be a force-flushing design or a non-force-flushing design, and when they are a force-flushing design, they are open momentarily as the main hydrant begins to open or just before the main hydrant valve closes so that full main pressure is applied to the drain passageway to clear the same before its next use.
Prior construction of drain valves for fire hydrants has been of the slide valve type, the spring loaded valve type, the sliding tube type, the cam operated to open and/or closed type or the like. Probably the most predominantly used type of drain valve is the slide valve type wherein a web-like structure having at least one longitudinal rib is carried by the main hydrant valve, the rib being provided with a resilient drain valve facing strip and arranged to cover or uncover the opening of the drain passageway to the interior of the valve seat ring above the main valve seat. For years the drain valve facing strip has been leather, but more recently rubber has been used and, even more recently, as disclosed in the co-pending application of Daniel A. Ellis and Joseph L. Daghe, Ser. No. 600,192 filed July 29, 1975 and entitled FIRE HYDRANT and assigned to the same assignee plastic drain valve strips such as polyethylene have been used.
In substantially all of the dry barrel hydrants, the automatic functioning drain valves are operated so that the drain passageway either provides communication between the exterior of the hydrant and the interior of the barrel or is closed and, further, the passageway remains open for draining purposes at all times the main hydrant valve is closed. Consequently, when a hydrant is located in an area that is damp or subject to being damp during certain periods of time or an area where the water table might rise at times above the position of the main hydrant valve in the hydrant, there is the opportunity of contaminated water or water with dirt, grit and other foreign matter backflowing into the hydrant valve where it may cause damage to the main hydrant valve mechanism or corrosion to the interior parts of the hydrant. Also in this construction, there is danger from subsequent freezing if the backflow pressure is sufficient to raise the water level in the barrel above the freezing line.
Many years ago an effort was made to solve the problem of backflow of water through the drain passageway into the barrel of a dry barrel fire hydrant and in this respect, a completely independent backflow preventer valve was installed in the drain passageway in series with a drain valve actuated by the fire hydrant's main valve. In this respect, the backflow preventer valve was a manually operable valve and it required utilizing a separate shaft extending from the backflow preventer valve element through the barrel, the shaft cooperating with a separate operating nut positioned on the bonnet of the fire hydrant. Thus, for the drain valve system of such a fire hydrant to operate successfully, it was necessary each time the fire hydrant was used to separately operate the operating nut for the main hydrant valve which in turn operated the drain valve and the operating nut to operate the backflow preventer valve. In more detail, when the fire hydrant was used, and the main hydrant valve was opened, it would close the drain valve so as to close the drain passage to the exterior of the hydrant. After the main hydrant valve was closed, thus, opening the drain valve, it was necessary to separately operate the backflow preventer valve to open the same so that the drain passage would be opened to the exterior. However, it does take considerable time for water to drain from the barrel to the exterior of the fire hydrant when the drain passageway had been freely opened, so therefore it required someone returning to the hydrant at a later time to close off the backflow preventer valve so as to prevent backflow of ground water into the barrel. Since the rate of flow of the drain water could vary due to different conditions of the ground surrounding the outlet of the drain passageway, sometimes the manually controlled backflow preventer valve was closed prior to full drainage of the hydrant barrel. In other instances careless operators failed to return and manually close the backflow preventer valve and therefore, the fire hydrant was subjected to the problem of backflow of ground water. Since such an arrangement required careful operation on the part of the operator as well as considerably more mechanism to be incorporated in the fire hydrant which would increase the cost of the same without insuring "fail proof" operation, this type of hydrant was never widely accepted and, as mentioned above, the predominant number of dry barrel fire hydrants commercially used today merely use a drain valve which automatically functions to operate in conjunction with the operation of the main hydrant valve and, thus, the hydrants are not protected from the backflow of ground water.